Two-band signal receiving system



W. L. CARLSON TWO-BAND SIGNAL RECEIVING SYSTEM Sept. 8, 1942.

Filed Aug. 6, 1941 Citforueg Patented Sept. 8, 1942 TWO-BAND SIGNAL RECEIVING SYSTEIVI Wendell L. Carlson, Haddonfieljd, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 6, 1941,Serial No.4i)5,*696

3 Claims.

This invention relates to a two-band signal receiving system which is responsive to signals in an ultra-high frequency band such as the signal receiving band between 42 and 50 Inc. and the low frequency or broadcast frequency hand between 530 and 1700 kc.

It is an object of this invention to provide an improved and simplified input circuit for a superheterodyne receiving system responsive to an ultra-high frequency and a low frequency signal receiving band and to provide variable tuning throughout each of said bands with a minimum number of tuning elements and with a single tuning control means.

In the construction of radio receiving systems, the tuning element may comprise a simple two or three-unit variable capacitor, the maximum number being three units for maintaining the overall size of the apparatus within limits'prescribed by modern design and to maintain a low production cost.

Accordingly, it is a further object of this invention to provide an improved and simplified input circuit for signal reception in an ultrahigh frequency and a low frequency signal receiving band which is variably tunable throughout each of said bands with a variable tuning capacitor having three tuning units.

It is a further object of this invention to provide an improved tuning system for a superheterodyne receiver responsive to two differing frequency bands which may be operated in the higher frequency or ultra-high frequency band with minimum cross modulation of received signals.

The invention will, however, be further understood from the following description, when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

Referring to the drawing, the figure is a schematic circuit diagram of a radio signal receiving system embodying the invention, in which 5 and 6 represent a dipole antenna connected through a transmission line I with the terminals 8 of a balanced primary winding 9 of an ultra high frequency input coupling transformer Ml. This transformer is provided with a balanced secondary H, the terminals i2 and it of which are connected with the signal input or control grids l4 and 15, respectively, of a balanced detector comprising suitable electric discharge tubes !6 and [1, which may be triodes as indicated.

The grids are provided with the usual coupling capacitors l8, and grid resistors L), which may be connected with any suitable source of biasing potential as indicated. The cathodes 2i! and 21 of the balanced detector are connected in parallel to ground 23 through a lead 22 and two seriesconnected cathode coupling windings 24 and 25, the latter having a shunt capacitor 26.

The anodes 28 and 29 of the balanced detector are connected to the terminals 38 and 3| of a balanced primary winding 32 of an intermediate frequency transformer 33,-the secondary of which is indicated at 34. Thelatter is tuned by a shunt capacitor 35 and a suitable magnetic tuning core 36 to a high intermediate frequency for the reception of signals in the ultra high frequency band for which the receiver isdesigned.

The detector anodes 28 and 29am connected in parallel relation to the primary 48 of a low frequency intermediate frequency transformer 4| through a connection with the midtap 42 of the balanced primary 32 and to a positive source of anode potential indicated by the lead 43. The alternating current path iscompleted to ground from the lead 43 through a bypass capacitor 44. The primary 4!] is suitably tuned to the low intermediate frequency by a shunt capacitor indicated at-45.

The secondarydfi of the low intermediate frequency transformer M is tuned to the'lower intermediate frequency by a shunt capacitor indicated at 41 and a suitable magnetic tuning core indicated at 43. Both primaries 32 and 48 may likewise be tuned by means of suitable magnetic tuning cores indicated at 49 and 5B.

The two secondaries 34 and 46 for the high and low intermediate frequency channels are serially connected with the signal input grid 5! of an intermediate frequency amplifier 52, the high frequency secondary 34 being connected more adjacent to the grid 55 or at the high potential side of the circuit While the secondary 4B is connected more adjacent to the bias supply connection indicated at 53.

The output anode'circuit 54 of the amplifier stage 52 is connected serially in circuit with two tuned primary windings 55 and 56 of a twochannel intermediate frequency amplifier which may be of any suitable type, although preferably constructed in accordance with Carlson Patent 2,167,605 which provides two amplifying channels with a single series of cascade-connected tubes arranged between the coupling transformers in a manner similar to that shown for the tube 52. 'The output transformers comprise tuned secondary windings 51 and 58 coupled to the primary windings 55 and 55, respectively,

and having a common output circuit indicated at 59.

The antenna, input primary 9 is provided with a center tap 68 which is connected to ground 8| through a low frequency primary winding 62 of a low frequency input coupling transformer 63, the secondary of which is indicated at E4. The latter is connected between ground 65 and a center tap 66 on the balanced secondary l l of the ultra high frequency input circuit and is variably tunable through a predetermined low frequency range by a variable shunt capacitor indicated at 61.

The cathode coupling windings 24 and 26 are coupled respectively to the oscillator circuits 10 and II, respectively, which are variably tunable in unison with the circuit 646'| by suitable means such as variable capacitors 12 and 13 connected in shunt with the tuning inductances I4 and 15 which are coupled respectively with the windings 24 and 25. The variable tuning means 61, 12 and 13 may be connected as indicated by the dotted line 16 for unitary control whereby the system may be tuned by single tuning means, which in this case may be the usual three-unit gang variable capacitor.

In the present example, the oscillators comprise oscillator tubes 80 and 8| coupled to the circuits H1 and H for generating oscillations in a high and a low frequency range for which the receiving system is designed, and are controlled selectively for operation in the two ranges by means of switches 82 and 83 in the cathode circuits 84. The present arrangement is such that the switch 82 is open when the switch 83 is closed, and vice Versa, to place one or the other of the oscillators in operation as a simple band changing means.

The oscillators may be any suitable type, and in the present example are of the feedback type having a control grid 85 connected with the tuned oscillator circuit and an anode 86 coupled to said tuned circuits through suitable coupling windings 81.

In the system shown, the dipole antenna -45 is connected with the balanced primary winding 9 which, in turn, is coupled through the balanced and broadly tuned secondary l l, with the balanced detector tubes l6 and I! which, in turn, transmit signals through the balanced primary winding 32 to the intermediate freqency amplifier 52.

Oscillator voltage is impressed on the balanced detector tubes I6 and I! through the cathode coupling winding 24 with the cathodes in parallel. This is the receiving circuit for ultra high frequency signals, such as signals in the 42 to 50 mo. band. For this frequency band, the low frequency winding 62 does not carry radio frequency currents and winding 64 is bypassed by capacity 61. On the intermediate frequency or output side of this circuit, the primary winding 40 has no R. F. or I. F. potentials impressed on it and the secondary winding 46 is bypassed by capacity 41.

For the broadcast or low frequency signal band, the antenna 5B operates as a T-type antenna supplying signals to the low frequency primary 62 through the high frequency primary 9 which offers substantially no impedance to signals in the low frequency or broadcast band. Signals from the primary 62 are applied to the tunable secondary 64 and to the detector grids l4 and I5 in parallel through the secondary H which offer substantially no impedance to signals in the low frequency band. The detector tubes [6 and I! operate to apply intermediate frequency signals to the low intermediate frequency primary 40 in parallel through the primary 32, and the intermediate frequency signals are applied to the amplifier 5| from the secondary 46, the high frequency primary and secondary offering substantially no impedance to signals in the low intermediate frequency band.

The oscillator applies voltage to the cathode circuit through the coupling winding 25, and as in the ultra high frequency band, the cathodes receive the oscillator voltage in parallel. The only switching required to change from one signal band to the other is to energize the oscillators 88 and 8| selectively by suitable switching means such as that shown in the present example.

By the simple circuit arrangement shown, the oscillator voltage is applied to the same cathode circuit and to the balanced detector cathodes in parallel for both the ultra high frequency and low frequency signal receiving bands. The pushpull or balanced ultra high frequency input circuit and output circuit for the detectors is effective to reduce cross-modulation of signals received through the ultra high frequency pass band circuits, while the tuning is accomplished by merely the oscillator tuning in the circuit 10. Furthermore, with this arrangement, a simple three-gang variable capacitor or its equivalent is all that is required for tuning the system variably through two differing frequency bands.

I claim as my invention:

1. A two-band signal receiving system comprising, in combination, a balanced detector comprising a pair of detector tubes having input and output electrodes and a common cathode connection therefor, a high frequency input circuit and a high intermediate frequency output circuit connected in balanced relation respectively to the input and output electrodes of said tubes, each of said circuits having a center tap, an oscillator coupled through said cathode connection to said tubes in parallel, a low frequency input circuit connected through the center tap on said first-named input circuit with the input electrodes of said tubes in parallel, a low intermediate frequency output circuit connected through the center tap on said first-named intermediate frequency output circuit with the output electrodes of said tubes in parallel, and a second oscillator coupled through said cathode connection to said tubes in parallel.

2. A two-band signal receiving system comprising, in combination, a detector having a balanced grid circuit, a high frequency input transformer having a broadly tuned secondary winding connected in balanced relation to said grid circuit, a high intermediate frequency balanced output circuit for said detector, an output transformer having a primary winding connected in balanced relation to said output circuit, a common cathode circuit for said detector, a high frequency oscillator coupled to said cathode circuit, a low frequency input circuit connected with said detector through the center connection on said secondary winding, a low intermediate frequency output circuit connected with said detector through the center connection on said primary winding, a low frequency oscillator coupled to said cathode circuit, and means for selectively energizing said oscillators.

3. A two-band signal receiving system comprising, in combination, a balanced detector having a pair of signal grids, a pair of cathodes and a pair of anodes, a high frequency input circuit connected in balanced relation to said grids, a high intermediate frequency output circuit connected in balanced relation to said anodes, an oscillator coupled to said cathodes in parallel, a

7 low frequency input circuit connected through said first-named circuit with said grids in parallel, a low intermediate frequency output circuit connected through said first-named intermediate 

